The invention relates to ion sources and to charge transfer, and more particularly to charge transfer ion sources.
The phenomenom of charge transfer, or electron charge exchange, has long been known. The simplest kind of charge transfer involves a collision between a neutral particle and a singly charged ion:
A++Bxe2x86x92A+B+
where A, B denote neutral particles in the ground state, and the superscript xe2x80x98+xe2x80x99 indicates a single positive charge state. In this case, ion B+ is created by an electron transfer from atom B to ion A+. Prior work on charge transfer in low pressure ( less than 100 mTorr) beam and plasma (gas discharge) systems deals mostly with collisions between single nuclei ion and single nuclei atoms. Some work has been done with simple molecules such as H2, O2, and CO. Charge transfer has been more generally applied to solid state devices and chemical systems, where charge transfer chemistry for very heavy molecules has been studied.
Even between simple atoms and molecules, charge transfer in ion beams and plasma, can be a complex process because ome reactants can be in excited states. Thus excited state charge transfer may occur, e.g.
A*++B*xe2x86x92A*+B*+
where the superscript xe2x80x98*xe2x80x99 indicates an excited state. For the present invention, it is assumed that the excited state is stable on a time scale that affects processes contributing to the species distribution in a plasma source.
If the reactants are like nuclei, resonant charge transfer can occur:
An++Axe2x86x92A+An+.
Resonant cross sections typically peak at or near zero relative energy, and can be significantly larger than nonresonant cross sections. Excited states seem to have relatively little effect on resonant charge transfer involving like atoms and nuclei.
Charge transfer between unlike reactants is usually nonresonant. However, resonant-like charge transfer can sometimes occur at low energy in unlike systems.
Charge transfer is usually a loss mechanism in ion beam systems. It can be an especially important effect in the low energy part of accelerators, such as the extraction gap, where peak gas pressure and maximum transfer cross section overlap. If the gas has molecular states, then charge transfer can produce molecular ions. As the molecular ions are accelerated, they can break up due to collisions with gas, producing breakup products with energy significantly different from the primary beam. Even in a source with pure atomic gas, charge transfer can produce some energy spread, because transfer product ions formed in the acceleration gap experience less than the full acceleration potential, and thus differ in energy from ions accelerated through the full gap.
Resonant and resonant-like charge transfers are intrinsic in plasma sources, because like ions and atoms are present in relatively high density and low relative energy (though often in excited states). The net effect on source performance is difficult to characterize since products and reactants are the same species.
Nonresonant transfer can also occur in plasma sources. However, this is usually less important because the relative energy of the reactants is far below the peak, and the peak cross section is much smaller than resonant.
An ion source for the production of Hxe2x88x92 ions based on a charge transfer mechanism has been previously proposed. Molecular hydrogen gas (H2) is dissociated into atomic hydrogen (H) using rf in a first chamber. The dissociated stream of H atoms is then introduced at the focus of a large area, biased, Hxe2x88x92 surface conversion ion source. The goal is to produce a high density of cold Hxe2x88x92 ions by charge transfer from a low density surface conversion Hxe2x88x92 source.
A particular application of plasma ion sources in the semiconductor industry is for ion implantation. Present ion implantation involves a single ion source chamber in which plasma (i.e., ion) generation occurs. Charge exchange between ions and neutral atoms is a natural process that occurs whenever ions and gases are mixed. In present ion sources, charge exchange is usually undesirable because it reduces the current density of the desired ion. Attempts to produce ions of heavy dopant molecules in standard ion sources have generally been unsuccessful because the energetic plasma electrons break up the molecules. In sources with a hot cathode, the cathode temperature can break up heavy molecules.
The trend is for semiconductor devices to become smaller and thinner. These smaller feature sizes challenge the ability of present systems to produce high beam current at low energy. Present ion implanters operate best at energies from about 20 keV to about 2 MeV. Future devices will require the same dopant current as present implanters, but at much lower energies, e.g. from about 2 keV down to hundreds of eV, to produce xe2x80x9cshallow junction implants.xe2x80x9d As energy levels are decreased to accommodate thinner devices, beam transport of standard dopant ions, e.g. boron (B+), arsenic (As+), and phosphorus (P+), becomes inefficient due to beam space charge.
The possibility of producing useful currents of heavy gas molecule ions offers significant efficiency gains over present implanters. For example, a decaborane ion (B10H14+) has ten dopant nucleons per charge, which provides two major benefits. The energy per dopant nucleon is less than one tenth of the ion energy, making it well suited for shallow junction doping or implantation. For example, a 10 keV beam of B10H14+ would deliver dopant at less than 1 keV per boron nucleon. Also the dopant current is ten times the ion current. Only 1 mA of B10H14+ is needed to deliver 10 mA of boron.
Thus it would be advantageous to provide an ion source which produces heavy ions with multiple dopant nucleons per ion, at a sufficient current density, to be effective as an ion implanter, particularly for shallow junction devices.
Other applications of such an ion source would be materials processing, where the macroscopic material properties are altered. This would include buried layers (high energy), or, surface growth/modification (low energy) with heavy molecular beams. Heavy molecular beams are sometimes called cluster beams. To date, practical research on cluster beam procesing has been hampered by the lack of a suitable ion source.
Accordingly it is an object of the invention to provide a charge exchange molecular ion source.
It is also an object of the invention to provide an ion source which produces molecular ions at a sufficient current to be effective as an ion implanter, particularly for shallow junction devices.
It is another object of the invention to provide a charge exchange ion source with low energy or high energy output for various applications, including surface modification and buried layers.
The invention is method and apparatus for producing ions, particularly molecular ions, including molecular ions with multiple dopant nucleons per ion, by charge exchange. The ion source contains a minimum of two regions separated by a physical barrier that utilizes charge exchange to enhance production of a desired molecular ion species. The essential elements are a plasma chamber for production of ions of a first species, a physical separator, and a charge transfer chamber where ions of the first species from the plasma chamber undergo charge exchange or transfer with the reactant atom or molecules to produce ions of a second species.
The invention can be implemented in a modified Bernas source to produce molecular ion generation by charge exchange, e.g. for semiconductor ion implantation. The Bernas source is modified to have two chambers, one a primary plasma chamber and the second a charge exchange chamber.
A particular embodiment of the invention produces decaborane ions. The ion source of the invention can produce heavy ions with multiple dopant nucleons so that the dopant nucleon energy is a fraction of the ion energy and the dopant nucleon current is a multiple of the ion current. Thus the ion source produces high current, low energy dopant beams which are suitable for shallow junction type devices.
For materials modification (cluster beam) processing, multiple process nucleons leverage the electrical beam current and reduce nucleon energy.